What is Belligerent?

Legal Definition
A belligerent (lat. bellum gerere, "to wage war") is an individual, group, country, or other entity that acts in a hostile manner, such as engaging in combat. Belligerent comes from Latin, literally meaning "one who wages war." Unlike the use of belligerent as an adjective to mean aggressive, its use as a noun does not necessarily imply that a belligerent country is an aggressor.

In times of war, belligerent countries can be contrasted with neutral countries and non-belligerents. However, the application of the laws of war to neutral countries and the responsibilities of belligerents are not affected by any distinction between neutral countries, neutral powers or non-belligerents. A non-belligerent may nevertheless risk being considered a belligerent if it aids or supports a belligerent in a way that is proscribed by neutral countries.

An interesting use of the term arose during the American Civil War, when the Confederate States of America, though not recognized as a sovereign state, was recognized as a belligerent power and so Confederate warships were given the same rights as US warships in foreign ports.
-- Wikipedia
Legal Definition
In international law. A term used to designate either of two nations which are actually in a state of war with each other, as well as their allies actively co-operating; as distinguished from a nation which takes no part in the war and maintains a strict indifference as between the contending parties, called a "neutral." U. S. v. The Ambrose Light (D. C.) 25 Fed. 412; Johnson v. Jones, 44 III. 151, 92 Am. Dec. 159.
-- Black's Law Dictionary
Legal Definition
Actually engaged in war.
-- Ballentine's Law Dictionary